With the rapid growth of data traffic, there are increasingly higher data transmission loads on carriers in licensed spectrums. Hence, it is an importation trend of evolution in the future development of Long Term Evolution (LTE) to offload the data traffic on the licensed carriers to carriers in unlicensed spectrums. The unlicensed spectrums have several advantages. People do not need to buy them—their spectrum resources are free or inexpensive. They have low admission requirements so both individuals and companies can deploy them. They include frequency bands at 5 GHz and 2.4 GHz and have a large bandwidth available. They have a characteristic of resource sharing, allowing several different systems or several different operators of one single system to operate therein, such that the spectrum utilization efficiency can be improved by means of resource sharing.
In view of the above advantages of the unlicensed spectrums, the LTE Release 13 has started a research since September 2014, including an important research topic regarding operations of the LTE system using carriers in the unlicensed spectrums. The techniques involved in this topic enable the LTE system to use the currently existing carriers in the unlicensed spectrums, so as to increase the potential spectrum resources of the LTE system significantly, allowing the LTE system to have a further reduced cost of spectrum resources. While bringing benefits for the development of the LTE system, the use of the carrier resources in the unlicensed spectrum causes a problem of fair coexistence between a Licensed-Assisted Access (LAA) system and other communication techniques (e.g., Wi-Fi). In addition, according to regulations in some areas, in order to access carriers in the unlicensed spectrum, a Listen Before Talk (LBT) mechanism is required to be performed first. That is, an LAA device, such as an evolved NodeB and/or a User Equipment (UE), has to fulfill the LBT requirement in those areas, in order to coexist friendly with other communication techniques (e.g., Wi-Fi).
With further researches in the LTE-U topic in the R13 LAA SI stage, finally in the first meeting of the WI stage (3GPP RAN1 #82), it has been agreed as to whether a UE needs to apply the LBT mechanism before an uplink transmission. That is, it is believed that the UE must apply the LBT mechanism individually before transmission, so as to improve the uplink performance of the system. Meanwhile, there are several types of candidate modes available for the uplink LBT and there are different configurations in on mode. Here, the candidates of uplink LBT modes include: LBT Category (Cat) 2, an LBT mechanism without random back-off; LBT Cat3, an LBT mechanism with random back-off and a fixed Contention Window (CW) size; and LBT Cat4, an LBT mechanism with random back-off and a variable CW size. Further, depending on different priority levels or requirements, some set of parameters for some LBT mechanism may be needed.
Hence, while some operational assumptions have been agreed currently for the LAA uplink, there are no solutions defined for switching between the candidates of the uplink LBT modes or between different sets of configuration parameters in one LBT mode. If an improper LBT mode is selected, e.g., when only one configured LBT mode or parameter set is adopted in an uplink system, its performance will be degraded. Furthermore, if an improper LBT mode is configured, there may be problems such as waste of resources allocated to a UE, waste of uplink indication information, low contention-based access rate, or unfairness.